Thermoplastic polyurethane elastomers (TPUs) are of industrial significance, since they exhibit excellent mechanical properties and can be processed by thermoplastic means inexpensively. Through the use of different chemical formation components, it is possible to vary their mechanical properties over a wide range. Comprehensive descriptions of TPUs, and the properties and uses thereof, can be found in Kunststoffe 68 (1978), p. 819-825 and Kautschuk, Gummi, Kunststoffe 35 (1982), p. 568-584.
TPUs are formed from linear polyols, usually polyester or polyether polyols, organic diisocyanates and short-chain diols (chain extenders). The formation reaction can be accelerated by additionally adding catalysts. The molar ratios of the formation components can be varied over a wide range, which allows the properties of the product to be adjusted. According to the molar ratios of polyols to chain extenders, products are obtained over a wide Shore hardness range. The thermoplastically processible polyurethane elastomers can be formed either stepwise (prepolymer method) or through the simultaneous reaction of all the components in one stage (one-shot method). In the prepolymer method, the polyol and diisocyanate are used to form an isocyanate-containing prepolymer which is reacted in a second step with the chain extender. The TPUs can be prepared continuously or batchwise. The best-known industrial production methods are the belt method and the extruder method.
As well as catalysts, auxiliaries and additives can also be added to the TPU formation components.
Particularly in the textile industry and in the construction industry, TPUs are used in the form of extruded films. In conjunction with textile layers or web-like fabrics, they find use in weather-resistant items of clothing, tarpaulins, roofing underlayment or exterior underlayment (composite components). The task of the TPU here in each case is firstly to act as a barrier to water in liquid form (e.g. rain) and secondly to release water in the gaseous state from the interior to the exterior, in order to obtain the most pleasant interior conditions possible. In contrast to other materials, TPU can fulfil this double function without production of micropores in the TPU layer.
Factors of crucial significance, as well as good water vapour permeability of such composite components and hence of the TPU layer, are both minimum swelling and adequate mechanical properties of the TPU films used in the composite component. An excessive tendency of the TPU film to swell increases the risk that the TPU film will become detached from the other layers that form part of the composite (e.g. the web), also called the substructure. As a result of this detachment, the usually very thin TPU films/layers are additionally exposed to the risk of damage in the form of cracks, for example. In addition, inadequate mechanical properties of the TPU film used not only make them more difficult to process to give the composite component, but likewise increase the risk of damage and lead to perceptible losses in the functionality of the components.
EP-A 1937743 describes TPUs having 30%-55% by weight of hard segment (diisocyanate and chain extender) and 45%-70% by weight of soft segment. The soft segment here comprises 20%-80% by weight of polyester diol and 20%-80% by weight of poly(oxypropylene)poly(oxyethylene) glycol-polyether diol, wherein the content of oxyethylene groups is less than 25% by weight of the polyether diol. The oxyethylene group content based on the totality of the polyols (soft segment) is thus below 20% by weight. Water vapour permeability is inadequate.
US-A 2008/0269454 discloses TPUs formed from diphenylmethane 4,4′-diisocyanate, 20%-60% by weight of hydrophilic polyether polyols comprising polyethylene glycol, polypropylene glycol or polytetramethylene glycol, having a carbon/oxygen ratio of 2:1 to 2.4:1, and 10%-40% by weight of aliphatic polyester polyols, some of which have good water vapour permeability and good processibility to films. However, the TPUs described here swell, or water vapour permeability is inadequate. In some cases, the raw material costs are also too high.
EP-A 1599522 is concerned with TPUs formed from a polyisocyanate, a symmetric chain extender, a further chain extender having a proportion in the total amount of chain extender of 1-50 mol % and having a molar ratio to the percentage by weight of the polyether polyol of 0.1 to 10, a polyester polyol and 1%-50% by weight, based on the total amount of polyol, of a polyether-co-polyol comprising polytetramethylene ether glycol, polypropylene oxide, poly(propylene oxide-co-ethylene oxide), polyethylene oxide or combinations thereof. The TPUs have the disadvantage of being subject to swelling, having a low water vapour permeability or having high raw material costs.
U.S. Pat. No. 3,493,634 describes TPUs formed from an aromatic diisocyanate and a mixture of 60-85 parts polyester, 15-40 parts poly(alkylene oxide) and 1.25-12.8 mol, based on one mole of polyester and poly(alkylene oxide), of an aliphatic glycol containing 2-12 carbon atoms. The mixture of polyester, poly(alkylene oxide) and glycol is to have a mean molecular weight of 300-450. The TPUs exhibit improved injection mouldability. However, the TPUs have the disadvantage of either having a low water vapour permeability or having high raw material costs.
U.S. Pat. No. 4,124,572 discloses TPUs formed from a poly(oxypropylene)-poly(oxyethylene) glycol containing 25%-60% by weight of oxyethylene groups, a polyester polyol, a polyisocyanate from the group consisting of toluene 2,4-diisocyanate, methylenebis(4-phenyl isocyanate), 4,4′-diisocyanatodicyclohexylmethane and isophorone diisocyanate, and also a chain extender, where the molar ratio of the chain extender to poly(oxypropylene)-poly(oxyethylene) glycol and polyester polyol is 3:1 to 10:1 and where the weight ratio of the poly(oxypropylene)-poly(oxyethylene) glycol to the polyester polyol is 10/90 to 90/10. The TPUs thus obtained, according to the description, have good mechanical and elastic properties, good thermal stability and colourability, and also good processibility and low costs. However, the water vapour permeability thereof is poor.